Production of hydrofluorocarbons

ABSTRACT

A process for the production of hydrofluorocarbons, in particular hydrofluoroalkanes such as difluoromethane, which comprises contacting an α-fluoro-ether, in particular a fluorinated dialkyl ether such as bis(fluoromethyl)ether in the liquid phase with a Lewis acid such as a fluoride of Nb, Sb, B, Ta, Al or Ti.

This is a continuation of Application No. 08/076,123 , filed on Jun. 14,1993, now abandoned, which was abandoned upon the filing hereof.

This invention relates to a process for the production ofhydrofluorocarbons and more particularly to a process for the productionof hydrofluoroalkanes.

In recent years chlorofluorocarbons, which are used on a large scalearound the world, have been perceived as having an adverse effect on theozone layer and/or as contributing to global warming.Chlorofluorocarbons are used, for example, as refrigerants, as foamblowing agents, as cleaning solvents and as propellants for aerosolsprays in which the variety of applications is virtually unlimited.Consequently, much effort is being devoted to finding suitablereplacements for chlorofluorocarbons which will perform satisfactorilyin the many applications in which chlorofluorocarbons are used but whichwill not have the aforementioned environmentally harmful effects. Oneapproach in the search for suitable replacements has centred onfluorocarbons which do not contain chlorine but which may containhydrogen, that is hydrofluorocarbons, of which many have been proposedas suitable replacements.

Several methods for the preparation of hydrofluorocarbons are known butmany of these methods involve the use of chlorine-containing startingmaterials and the production of chlorine-containing by-products.

In our co-pending UK Patent Application No. 9126355.8 there is describeda chlorine-free process for the production of hydrofluorocarbons whichcomprises heating an α-fluoroether to elevated temperature in the vapourphase.

We have now found that α-fluoroethers may also be caused to decompose inthe liquid phase to yield hydrofluorocarbons.

According to the present invention there is provided a process for theproduction of a hydrofluorocarbon which comprises contacting anα-fluoro-ether in the liquid phase with a Lewis acid.

The process of the present invention is especially useful in theproduction of hydrofluoroalkanes and according to a further aspect ofthe invention there is provided a process of producing ahydrofluoroalkane having the formula C_(n) H_(x) F_(y) in which n is aninteger from 1 to 6, y is an integer of at least 2 and x=2n+2-y, whichcomprises contacting an α-fluoroether in the liquid phase with a Lewisacid.

In the hydrofluoroalkane of formula C_(n) H_(x) F_(y), n is preferablyan integer from 1 to 4 and y is preferably an integer from 2 to 9; morepreferably n is 1 or 2, and y is an integer from 2 to 5. Where n is 1, yis especially preferably 2, and the hydrofluoroalkane product of theinvention may be difluoromethane, di-, tri-, tetra- or penta-,fluoroethane. We especially prefer to employ the process of theinvention as a process of producing difluoromethane,1,1,1,2-tetrafluoroethane and pentafluoroethane.

By an α-fluoro-ether there is meant an ether having a fluorine atomattached to a carbon atom at the α-position relative to the oxygen atom,that is an ether containing the group --C--O--CF--. A particularlyuseful class of ethers is that having the general formula R--O--CF--R¹R², wherein R, R¹ and R² are as hereinafter defined.

We have found that these α-fluoro-ethers of formula R--O--CF--R¹ R² maybe caused to breakdown in the liquid phase upon contact with a Lewisacid to yield hydrofluorocarbons R--F.

In the ethers of formula R--O--CF--R¹ R², the group R may generally takeany form and may comprise heteroatoms, for example O, S or N, providedthat it comprises at least one carbon atom. The group R may be forexample saturated or unsaturated, linear or branched chain, cyclic oracyclic, aliphatic or aromatic.

However, the process of the present invention is, as previously stated,useful in particular for the production of hydrofluoroalkanes from theclass of ethers in which the R group is an optionally substituted alkylgroup which may comprise one, two or even more carbon atoms, say up to 6or even more carbon atoms. The alkyl group R will usually be a straightchain alkyl group although it may also be a branched chain alkyl group.The group R may comprise only carbon and hydrogen although usually thegroup R will be a fluorinated group.

The α-fluoro ether will typically be an α-fluoroalkyl ether, that is anether of formula R--O--CF--R¹ R² wherein R¹ and R² are hydrogen,fluorine or optionally substituted alkyl groups which may comprise one,two or even more carbon atoms, say up to 6 or even more carbon atoms.The alkyl groups R¹ and R² will usually be acyclic straight chain alkylgroups although they may also be acyclic branched chain alkyl groups orcyclic alkyl groups. The groups R¹ and R² may comprise only carbon andhydrogen although usually the groups R¹ and R² will be fluorinatedgroups. Typically at least one of R¹ and R² will be a hydrogen atom.Preferably neither of R¹ and R² will be a fluorine atom.

Thus, according to a preferred embodiment of the invention there isprovided a process for the production of hydrofluoroalkanes whichcomprises contacting an α-fluoroether having the formula R--O--CF--R¹ R²wherein R is an optionally substituted alkyl group comprising from 1 to6 carbon atoms and R¹ and R² are H, F or optionally substituted alkylgroups containing from 1 to 6 carbon atoms, in the liquid phase with aLewis acid. Preferably the group R also contains at least one fluorineatom; and R¹ and R² are not F.

The α-fluoro-ether is preferably an α-fluoromethyl-ether, R--O--CFH₂, ora tetrafluoroethyl ether R--O--CFH--CF₃, since these α-fluoro-ethers arereadily prepared and on contact in the liquid phase with a Lewis acidyield particularly useful hydrofluoroalkanes.

The α-fluoromethyl-ether may be, for example, FCH₂ --O--CH₂ Fbis(fluoromethyl)ether, FCH₂ --O--CH₃ fluoromethyl-methyl ether, FCH₂--O--CH₂ CF₂ H 1,1,-difluoroethyl-fluoromethyl ether; or FCH₂ --O--CH₂CF₃ 1,1,1-trifluoroethyl-fluoromethyl ether, which when contacted in theliquid phase with a Lewis acid may decompose to yield the followinghydrofluoroalkanes respectively, CH₂ F₂, CH₃ F, CHF₂ CH₂ F and CF₃ CH₂F. The tetrafluoroethyl ether may be, for example, CF₃ CHF--O--CH₂ CF₃,which upon contact in the liquid phase with a Lewis acid may yield1,1,1,2-tetrafluoroethane, or CF₃ CFH--O--CFHCF₃ or CF₃ CHF--O--CH₂ Fwhich upon contact in the liquid phase with a Lewis acid may yield CF₃CF₂ H.

According to a first preferred embodiment of the invention there isprovided a process for the production of 1,1,1,2-tetrafluoroethanecomprising contacting CF₃ CHX--O--CFR¹ R² in which X is H or F and R¹and R² are H, F or an optionally substituted alkyl group containing from1 to 6 carbon atoms, in the liquid phase with a Lewis acid. The ether ispreferably FCH₂ --O--CH₂ CF₃ and/or CF₃ CHF--O--CH₂ CF₃.

According to a second preferred embodiment of the invention there isprovided a process for the production of pentafluoroethane comprisingcontacting CF₃ CHF--O--CFR¹ R² in which R¹ and R² are as defined in theprevious paragraph, in the liquid phase with a Lewis acid. The ether ispreferably CF₃ CFH--O--CFHCF₃ and/or CF₃ CFH--O--CH₂ F.

According to a third preferred embodiment of the invention there isprovided a process for the production of difluoromethane comprisingcontacting an α-fluoroether having the formula CH₂ F--O--CFR¹ R² inwhich R¹ and R² are as defined in the first preferred embodiment of theinvention, in the the liquid phase with a Lewis acid. The ether ispreferably bis(fluoromethyl)ether.

The term "Lewis acid" is commonly known and used by those skilled in theart, and any Lewis acid, for example AlCl₃, may be employed in theprocess of the invention. We have found that materials which are notLewis acids, for example, the Bronsted acids such as nitric,trifluoroacetic, sulphuric, fluorosulfonic and trifluoromethanesulphonic acids; and other materials such as KF, MnF₃ and glass are noteffective in the production of hydrofluorocarbons from α-fluoroethers.Furthermore, materials which are known to be effective for the liquidphase decomposition of alkyl fluoroformates, for example quaternaryammonium salts such as tetrabutyl ammonium fluoride, have no utility inthe liquid phase α-fluoroether decomposition process of the presentinvention.

Particularly suitable Lewis acids for use in the process of theinvention contain fluoride as the anionic species, since where anionicspecies other than fluoride are present, in particular halides otherthan fluoride, e.g. chlorides, many undesirable by-products may beproduced. However anionic species other than fluoride, for examplehalide other than fluoride, alkoxide, etc, do result in the productionof hydrofluorocarbons and may be employed if desired. Preferred Lewisacids include the fluorides of elements, in particular metals, of GroupIII (a or b), IV (a or b) and V (a or b) of The Periodic Table of theElements, for example AlF₃, BF₃, SnF₄, TaF₅, TiF₄,NbF₅ and SbF₅.

We particularly prefer to employ Lewis acids in which the centralcation, usually a metal, has a charge/radius ratio of at least 5.0 andpreferably at least 6.0. We especially prefer to employ SbF₅, BF₃, NbF₅and/or TiF₄ in the process of the invention. Mixtures of Lewis acids maybe employed, if desired.

The Lewis acid may be generated in situ, for example by employing thecorresponding halides other then fluorides, for example chlorides, oroxides and a source of fluoride, for example hydrogen fluoride. They mayalso be generated in situ by employing the metal itself and a source offluoride, especially hydrogen fluoride.

The process may be conducted in the presence or absence of hydrogenfluoride. We prefer to conduct the process in the presence of hydrogenfluoride. The amount of hydrogen fluoride employed may vary within awide range but generally a stoichiometric excess of hydrogen fluoride tobis(fluoromethyl)ether is preferred. The molar ratio ofbis(fluoromethyl)ether to hydrogen fluoride may be in the range fromabout 2:1 to about 1:50, preferably in the range from about 1:2 to about1:20. The hydrogen fluoride may serve not only to improve the conversionof the ether and the selectivity to difluoromethane but also toregenerate the Lewis acid thereby rendering the process of the inventioncatalytic.

The process is preferably conducted under substantially anhydrousconditions, since many of the Lewis acids are readily hydrolysed.However, the susceptibility of any particular Lewis acid to hydrolysisby water varies with the particular Lewis acid employed and it is notessential that the process is conducted under substantially anhydrousconditions; indeed certain Lewis acids may be employed in the form oftheir hydrates, for example BF₃.

The process is carried out under conditions of temperature and pressuresuch that the α-fluoroether is in the liquid phase. Preferred conditionsof temperature and pressure are such that the α-fluoroether is in theliquid phase and the hydrofluorocarbon product is in the vapour phase,as the hydrofluorocarbon product of the process may then easily separatefrom the reaction mixture. However the hydrofluorocarbon product mayalso be liquid under the conditions of the process, if desired. Thus,the particular conditions of temperature and pressure employed will bedependent to some extent upon the particular ether employed. Generally,the temperature will be in the range from about -50° C. to about 300° C.depending to some extent upon the pressure employed and preferably inthe range from about -30° C. to about 200° C., more preferably fromabout -20° C. to about 150° C. Where atmospheric pressure operation isemployed the temperature will usually be in the range from about -20° C.to about 100° C. Atmospheric pressures are conveniently employedalthough superatmospheric pressure or subatmospheric pressure may beemployed if desired.

Processes are known for the production of at least some specificα-fluoro-ethers and any of these known processes may be used for theproduction of the α-fluoro-ether starting materials in the presentinvention. Thus, for example, the α-fluoro-ether may be produced asdescribed in The Journal of Inorganic Nuclear Chemistry-32, (1970),1748, The Journal of the American Chemical Society 82 (1960) 543, or TheJournal of Organic Chemistry, 28, 492 (1963).

However, we have found that a particularly convenient, and thuspreferred, general method for the production of the α-fluoro-ether is byreacting a non-enolisable aldehyde with hydrogen fluoride, preferably inthe liquid phase, and in the presence of an alcohol.

According to a preferred embodiment of the invention there is provided aprocess for the production of a hydrofluorocarbon which comprises (a)contacting a non-enolisable aldehyde with hydrogen fluoride in theliquid phase in the presence of an alcohol to produce an α-fluoro-etherand (b) contacting the α-fluoro-ether in the liquid phase with a Lewisacid.

A non-enolisable aldehyde is required in order that the aldehyde is notpolymerised in hydrogen fluoride when the two are reacted together.

The non-enolisable aldehyde employed is preferably formaldehyde ortrifluoroacetaldehyde since these aldehydes are the most readilyavailable non-enolisable aldehydes and they yield the most useful finalhydrofluorocarbons: formaldehyde is particularly preferred. Indeed, in afurther preferred embodiment of the invention, both formaldehyde andtrifluoroacetaldehyde are reacted with hydrogen fluoride to produce amixture of CF₃ CFH--O--CH₂ F and CH₂ F--O--CH₂ F. This mixture may thenbe converted to hydrofluoroalkanes, or a separate alcohol may then beadded to this mixture to produce further α-fluoroethers.

Production of the α-fluoroether may be conveniently effected simply bydissolving the non-enolisable aldehyde in any of its readily availableforms in liquid hydrogen fluoride at about room temperature, in thepresence of an alcohol.

The non-enolisable aldehyde may be provided in any of its known forms.Thus formaldehyde may be provided, for example, in one of its polymericforms, paraformaldehyde or trioxane, or in its monomeric form which maybe provided, for example, from a process stream in which it has beenfreshly made, for example by the oxidation of methanol.Trifluoroacetaldehyde may be provided, for example, in its hydrated formCF₃ CH(OH)₂ or in its dehydrated form CF₃ CHO.

Accordingly, whenever used herein, the term non-enolisable aldehyde isto be understood as including non-enolisable aldehydes in any of theirknown forms.

In general, where formaldehyde is used as the non-enolisable aldehyde, apolymeric form of formaldehyde such as paraformaldehyde is preferredwhere the formaldehyde is dissolved in liquid hydrogen fluoride.Paraformaldehyde and trioxane dissolve readily in liquid hydrogenfluoride and the production of the α-fluoro-ether may be convenientlycarried out by dissolving paraformaldehyde or trioxane in liquidhydrogen fluoride at about room temperature and at about atmosphericpressure in the presence of an alcohol.

The molar ratio of the non-enolisable aldehyde to hydrogen fluoride mayvary considerably, for example in the range about 1:0.5 to 1:50 but ingeneral a stoichiometric excess of hydrogen fluoride is preferred.Typically, the molar ratio of non-enolisable aldehyde to hydrogenfluoride will be in the range about 1:2 to about 1:10.

The reaction of the non-enolisable aldehyde with hydrogen fluoride iscarried out in the presence of an alcohol. The alcohol may be generatedin situ. Thus, the reaction of the non-enolisable aldehyde, for exampleformaldehyde or trifluoroacetaldehyde, with hydrogen fluoride isbelieved to yield an intermediate alcohol FCH₂ OH and CF₃ CHFOHrespectively which may then condense to give the α-fluoro-ether FCH₂--O--CH₂ F and CF₃ CFH--O--CFHCF₃ respectively.

Alternatively a wider range of α-fluoro-ethers may be produced by addinga separate alcohol. Where a separate alcohol is added, it may be addedat the same time as the hydrogen fluoride and non-enolisable aldehyde,or it may be added subsequently to the mixture of aldehyde and hydrogenfluoride. Furthermore the alcohol may be first added to the hydrogenfluoride and the aldehyde may then be added to this reaction mixture.Thus the order of addition of the hydrogen fluoride, aldehyde andalcohol is not critical.

Where the alcohol is added separately, the alcohol may have the generalformula R--OH provided that the alcohol must be inert to hydrogenfluoride and the α-fluoro-ether. The group R may become the R group ofthe ether produced having the general formula R--O--CF--R¹ R². Thegroups R, R¹ and R² are as hereinbefore defined.

Whilst production of the α-fluoroether is not limited by theory and thefollowing theory is given merely by way of explanation, the provision ofa separate alcohol effectively leads to a transetherification with theether produced by condensation of two molecules of the alcohol believedto be generated in situ by the reaction of hydrogen fluoride with thenon-enolisable aldehyde. Thus, as previously described where a separatealcohol is not added to hydrogen fluoride and formaldehyde, twomolecules of the transient intermediate FCH₂ OH condense to give CH₂F--O--CH₂ F. Where a separate alcohol is present one of the --CH₂ Fgroups is effectively substituted by the group R of the separate alcoholwhich is present. This may occur by way of transetherification of thealcohol R--OH with CH₂ F--O--CH₂ F, or by condensation of FCH₂ OH withR--OH. However, the precise mechanism is not important as the effectivefinal ether produced is the same.

The group R may generally take any form provided that it comprises atleast one carbon atom, and the group R may for example be saturated orunsaturated, linear or branched chain, cyclic or acyclic, aliphatic oraromatic. The group R may also comprise heteroatoms, for example O, S orN.

However, the process of this further preferred embodiment of the presentinvention is useful in particular for the production of ethers in whichthe R group is an optionally substituted alkyl group which may comprisesone, two or even more carbon atoms, say up to 6 or even more carbonatoms. The alkyl group R will usually be a straight chain alkyl groupalthough it may also be a branched chain alkyl group. The R group maycomprise only hydrogen and carbon, for example the R group may be CH₃,C₂ H5. Preferably however, the R group will be fluorinated, for examplethe R group may be FCH₂ CH₂ --, HCF₂ CH₂ --, CF₃ CH₂ --, (CF₃)₂ CH--, orCF₂ HCF₂ CH₂ --. Thus the alcohol which is added is preferably a primaryalcohol and may comprise such R groups, for example the alcohol may bemethanol, ethanol, 2-monofluoroethanol, 2,2-difluoroethanol,2,2,2-trifluoroethanol, hexafluoroisopropanol or1,1,2,2-tetrafluoropropanol. Some at least of the alcohols may begenerated in situ by adding an epoxide to the non-enolisablealdehyde/hydrogen fluoride mixture. Thus for example,2-monofluoroethanol may be generated in situ by the addition of ethyleneglycol which reacts with hydrogen fluoride to produce2-monofluoroethanol.

Where the alcohol is added separately, it may be added in similarproportions as the non-enolisable aldehyde, that is, in the molar ratioof alcohol to hydrogen fluoride for example in the range about 1:0.5 to1:50 but in general a stoichiometric excess of hydrogen fluoride ispreferred. The proportion of alcohol added may also depend upon theparticular alcohol used since we have found that with certain alcohols,the addition of too great a proportion of the alcohol leads to theformation of an undesirable acetal rather than the requiredα-fluoroether. Typically the molar ratio of alcohol to hydrogen fluoridewill be in the range about 1:2 to about 1:10.

The α-fluoro-ether may be isolated from the aldehyde and hydrogenfluoride, from which it is produced, and any by-products, before theα-fluoro-ether is contacted in the liquid phase with a Lewis acid. Theether may be isolated, for example, by adding alkali to thenon-enolisable aldehyde/hydrogen fluoride/alcohol liquid mixture andheating the resulting alkaline solution, for example up to about 50° C.,in order to drive the α-fluoro-ether off. Alternatively theα-fluoroether may conveniently be isolated by contacting the productstream with water at a temperature in the range from about 50° C. toabout 80° C. The α-fluoro ether may then be collected in a cold trap orpassed directly to the heating zone.

We especially prefer that the α-fluoroether and optionally hydrogenfluoride are separated from water which is also produced by the reactionof the non-enolisable aldehyde with hydrogen fluoride. Thus theα-fluoroether and optionally hydrogen fluoride are preferably contactedin the liquid phase with a Lewis acid in the substantial absence ofwater. Preferably the α-fluoroether and optionally hydrogen fluoridewhich is contacted with the Lewis acid contains less than 5% by weightwater, more preferably less than 1% by weight and especially less than0.5% by weight water, although the use of certain Lewis acids, inparticular BF₃, may allow the process of the invention to be performedwithout a loss in selectivity to the desired product in the presence oflager quantities of water.

In particular, we have achieved high conversions ofbis(fluoromethyl)ether and high selectivities to difluoromethane withmany Lewis acids where the bis(fluoromethyl)ether/hydrogen fluoridemixture which is contacted with the Lewis acid contains from about 500ppm to about 3000 ppm of water.

Separation of the α-fluoroether and optionally hydrogen fluoride fromwater may be achieved in any suitable manner, and conveniently forexample by vaporising the α-fluoroether and optionally hydrogen fluoridefrom the product mixture obtained by reacting a non-enolisable aldehydewith hydrogen fluoride in the presence of an alcohol, or by contactingthe product mixture with a solid drying agent. Thus, for example astream of an inert gas, for example nitrogen may be sparged through thesolution of α-fluoroether and hydrogen fluoride (and other by-products).

Accordingly, in a further embodiment of the invention there is provideda process for the production of a hydrofluoroalkane which comprises thesteps of (a) reacting a non-enolisable aldehyde with liquid hydrogenfluoride in the presence of an alcohol to produce an α-fluoroether, (b)separating at least some water from the product of step (a) and (c)contacting the α-fluoro-ether and optionally hydrogen fluoride in theliquid phase with a Lewis acid.

The production of the especially preferred α-fluoroether,bis(fluoromethyl)ether for use in the process of the present inventionis described in our published European Patent Application No. 0 518 506,the contents of which are incorporated herein by reference in so far asthey relate to the production of bis(fluoromethyl)ether.

The invention is illustrated, but not limited, by the followingexamples.

EXAMPLE 1

3.2 g of bis(fluoromethyl)ether were charged to a 30 ml FEP (copolymerof hexafluoropropylene and tetrafluoroethylene) bottle, a septum cap wasfitted and the bottle was cooled in ice to 0° C. 0.3 g of SbF₅ wereinjected into the bottle, the bottle was shaken and the head space abovethe liquid was analysed by gas chromatography, mass spectrometry andinfra-red spectroscopy. The results, based on integration of the peaksin the gas chromatograph are shown below:

    ______________________________________    HEAD SPACE    PRODUCT          % (v/v)    ______________________________________    CH.sub.2 F.sub.2 8.16    CH.sub.2 O       0.16    CH.sub.2 F--O--CH.sub.2 F                     91.6    Others           0.08    ______________________________________

EXAMPLE 2

The procedure of example 1 was repeated except that 0.5 g of aluminiumchloride was added to 2.0 g of bis(fluoromethyl)ether. The results ofanalysis of a sample taken from the head space, based on gaschromatagram peak areas, are given below.

    ______________________________________    HEAD SPACE    PRODUCT             % (v/v)    ______________________________________    CH.sub.2 F.sub.2    0.05    CH.sub.3 --O--CH.sub.2 F                        0.02    CH.sub.2 F--O--CH.sub.2 F                        97.6    CH.sub.2 F--O--CH.sub.2 Cl                        2.2    CH.sub.2 F--O--CH.sub.2 --O--CH.sub.2 F                        0.13    ______________________________________

0.2 mls of anhydrous hydrogen fluoride were then added to the mixture inthe FEP bottle and the headspace was reanalysed. The results are shownbelow:

    ______________________________________    HEAD SPACE    PRODUCT             % (v/v)    ______________________________________    CH.sub.3 F          0.01    CH.sub.2 F.sub.2    0.2    CH.sub.3 --O--CH.sub.2 F                        0.06    CH.sub.2 F--O--CH.sub.2 F                        87.0    CH.sub.2 F--O--CH.sub.2 Cl                        12.53    CH.sub.2 F--O--CH.sub.2 --O--CH.sub.2 F                        0.1    CH.sub.2 Cl.sub.2   0.1    ______________________________________

The following examples 3 to 23 were conducted in a 125 ml Hastelloy `C`autoclave.

EXAMPLE 3

21.0 g of bis(fluoromethyl)ether (containing about 1000 ppm of water)and 0.7 g of NbF₅ were charged to the autoclave and the autoclave washeated to a maximum of 186° C. over a period of 3 hours. After this timethe volatile organic products were separated from the catalyst/residuesby distillation and the organics were analysed by Gas Chromatograhy,Infra-red spectroscopy and Mass Spectrometry. The results are shownbelow:

    ______________________________________    Product         Yield (%)    ______________________________________    Difluoromethane 10.8    Methyl fluoride 34.1    ______________________________________

EXAMPLE 4

The procedure of example 3 was repeated except that the maximumtemperature was 84° C. The results are shown below:

    ______________________________________    Product         Yield (%)    ______________________________________    Difluoromethane 2.9    Methyl fluoride 9.4    ______________________________________

EXAMPLES 5 to 13

The procedure of example 3 was repeated with the Lewis acids stated inTable 1. The presence and amount thereof of hydrogen fluoride; thecatalyst employed and the maximum temperature are given in Table 1.

                  TABLE 1    ______________________________________    Lewis acid.             HF      BFME    Max Temp.                                      Yield (%).    (g)      (g)     (g)     (°C.)                                      CH.sub.2 F.sub.2                                            CH.sub.3 F    ______________________________________    SbF.sub.5             52.3    41.0    101      45.8  7.5    4.2    TiF.sub.4             25.7    20.6    141      46.2  49.4    7.0    AlF.sub.3             26.2    20.6    162      4.67  58.3    5.0    TaF.sub.5             23.4    20.5    130      60.5  35.9    7.0    NbF.sub.5             25.3    20.5    125      55.0  25.6    7.0    NbF.sub.5             23.5    20.5    50       61.8  14.0    5.0    CsF      24.5    20.0    161      9.4   0.8    7.0    BF.sub.3 32.2    18.1     65      46.6  18.3    1.6    BF.sub.3.2H.sub.2 O             361.5   361.5    50      10.2  84.2    96    ______________________________________

EXAMPLE 14

The procedure of example 3 was repeated except that 18 g offluoromethyl-2,2,2-trifluoroethyl ether, 27.5 g of hydrogen fluoride and5 g of TaF₅ were heated to a maximum of 178° C. The results are shownbelow:

    ______________________________________    Product.          Yield (%).    ______________________________________    Difluoromethane   40.0%    1,1,1,2-tetrafluoroethane                      5.0%    ______________________________________

COMPARATIVE EXAMPLES 1 to 3

The procedure of example 3 was repeated except that a Lewis acid was notpresent. The amounts of BFME and HF present are given in Table 2 below.In comparative example 3 various materials which were not Lewis acidswere employed, as stated.

                  TABLE 2    ______________________________________    HF    BFME    Other    Max Temp.                                    Yield (%).    (g)   (g)     (g)      (°C.)                                    CH.sub.2 F.sub.2                                           CH.sub.3 F    ______________________________________    0.0   26.0    None     200      0.2    0.1    25.4  20.5    None     155      6.8    72.1    24.5  20.0    (a) MnF.sub.3                  (b) Glass     Qualitative: No                  (c) KF        hydrofluorocarbon product                  (d) Bu.sub.4 NF    ______________________________________

EXAMPLE 15 (a) Preparation of Fluoromethyl-2,2,3,3-tetrafluoropropylether.

400 g of anhydrous hydrogen fluoride were added to 80 g of trioxane at0° C. and to this mixture 160 g of tetrafluoropropanol were added withcooling. The resulting mixture was poured onto ice and the lower organiclayer was separated from the aqueous layer. The organic layer collectedwas dried and purified by vacuum distillation to give an organicfraction having the following composition:

    ______________________________________    Component                (%)    ______________________________________    Fluoromethyl-2,2-3,3-tetrafluoropropyl ether                             87.0    CHF.sub.2 CF.sub.2 CH.sub.2 --O--CH.sub.2 --O--CH.sub.2 F                             11.25    Bis(fluoromethyl)ether   1.75    ______________________________________

(b) Preparation of 1,1,2,2,3-pentafluoropropane

19.2 g of the composition prepared in (a) was charged to a Hastelloyautoclave together with 17.8 g of anhydrous hydrogen fluoride and 2 g ofNbF₅. The mixture was heated to a maximum of 85° C. for 16 hours. Thevolatile organic products were distilled from the autoclave and wereanalysed b₇ gas chromatography and mass spectrometry. The composition ofthe volatile organic fraction collected is given below:

    ______________________________________    Component.               (%)    ______________________________________    Fluoromethyl-2,2-3,3-tetrafluoropropyl ether                             69.3    1,1,2,2,3-pentafluoropropane                             23.7    Difluoromethane          4.3    Methyl fluoride.         2.7    ______________________________________

EXAMPLE 16 (a) Preparation of fluoromethyl-2,2,2-trifluoroethyl ether.

20 g of trioxane was added to 100 g of anhydrous hydrogen fluoride withstirring and cooling and to the mixture was added 50 g of2,2,2-trifluoroethanol at 0° C. The resulting mixture was poured ontoiced water. The lower organic layer was separated from the aqueous layerand the organic layer was analysed by Gas chromatography, Infra-redspectroscopy and Mass spectrometry. The organic layer had the followingcomposition:

    ______________________________________    Component              (%)    ______________________________________    fluoromethyl-2,2,2-trifluoroethyl ether                           91.6    Bis(fluoromethyl)ether 5.6    CF.sub.3 CH.sub.2 --O--CH.sub.2 --O--CH.sub.2 F                           2.8    ______________________________________

(b) Preparation of Difluoromethane and 1,1,1,2-tetrafluoroethane.

18.9 g of the composition from (a) was charged to a Hastalloy autoclavetogether with 19 g of anhydrous hydrogen fluoride and 2 g of NbF₅ andthe mixture was heated to 100° C. for 2 hours. The volatile organicswere distilled from the autoclave and analysed by Mass spectrometry. Thecomposition of the organics collected was as follows:

    ______________________________________    Component               (%)    ______________________________________    fluoramethyl-2,2,2-trifluoroethyl ether.                            29.0    CF.sub.3 CH.sub.2 --O--CH.sub.2 --O--CH.sub.2 F                            27.6    CF.sub.3 CH.sub.2 --O--CH.sub.2 --O--CH.sub.2 F                            13.6    CF.sub.3 CH.sub.2 --O--CH.sub.3                            11.7    Difluoromethane         7.5    Methyl fluoride         5.9    Bis(fluoromethyl)ether  2.7    1,1,1,2-tetrafluoroethane                            2.0    ______________________________________

EXAMPLE 17

43.7 g of fluoromethyl-2,2,2-3,3,3-hexafluoroisopropylether, 28.7 g ofhydrogen fluoride and 2.8 g of NbF₅ were charged to a Hastelloyautoclave and heated to a maximum of 50° C. for 16 hours. The volatileorganic products were distilled from the autoclave and analysed by Gaschromatography. The composition of the organics collected was asfollows:

    ______________________________________    Component                   (%)    ______________________________________    Difluoromethane             60.0    Fluoromethyl-2,2,2-3,3,3-hexafluoroisopropylether                                34.0    1,2,2,2,3,3,3-heptafluoroisopropane                                0.1    Others                      5.9    ______________________________________

EXAMPLES 18 to 23

In the following examples the procedure of example 3 was followed exceptthat the Lewis acid was generated in situ from the metal or oxidethereof as stated in Table 3 and the amounts of bis(fluoromethyl)ether,hydrogen fluoride and catalyst charged to the autoclave were 20 g, 25 gand 2 g respectively. The results and conditions are shown in Table 3.

                  TABLE 3    ______________________________________              Max   Conversion   Selectivity    Metal           Temp    CH.sub.2 F--O--CH.sub.2 F                                       CH.sub.2 F.sub.2                                             CH.sub.3 F    (or oxide)            Form    (°C.)                            (%)        (%)   (%)    ______________________________________    Hastelloy            Vessel  155     35.5       5.4   29.9    Antimony            Shot    100     68.2       18.4  64.3    Niobium Turn-   100     76.9       37.7  47.5            ings    Tantalum            Wire    100     99.6       20.8  77.1    Tungsten            Wire    100     92.5       14.4  83.5    Fe.sub.2 O.sub.3            Pow-    125     89.9       8.2   89.6            der    ______________________________________

EXAMPLE 24

600 g of a 50/50% w/w mixture of bis(fluoromethyl)ether and hydrogenfluoride containing 3000 ppm (by weight) water was charged to a 1 litreHastelloy autoclave at room temperature (19° C.). BF₃ was then chargedto the closed autoclave to a pressure of 5 barg (approx 10.6 g BF₃). Thepressure dropped to 4.2 barg as BF₃ was absorbed into the liquid, thepot was heated to 50° C. and the pressure rose to 6.6 barg. Five vapoursamples were taken from the autoclave headspace at regular intervalsover a period of 6 hours whilst the temperature was maintained at 50° C.After each vapour sample was taken, BF₃ was charged to the autoclave tomaintain the pressure at about 7 barg (approx. 1-2 g BF₃). The vapoursamples were analysed by gas chromatography. Over the 6 hour period thevapour was found to comprise 94.7% by volume difluoromethane and 4.6%methyl fluoride.

We claim:
 1. A process for the production of difluoromethane whichcomprises contacting the compound CH₂ F--O--CFR¹ R² in which R¹ and R²are each H, in the liquid phase with a Lewis acid.
 2. A process asclaimed in claim 1 in which the Lewis acid comprises a fluoride of ametal.
 3. A process as claimed in claim 2 in which the metal ion of themetal fluoride has a charge/radius ratio of at least 5.0.
 4. A processas claimed in claim 3 in which the Lewis acid is selected from thefluorides of niobium, antimony, boron, titanium, tantalum, aluminium andtungsten.
 5. A process as claimed in claim 3 which the metal fluoride ismixed with the α-fluoroether.
 6. A process as claimed in claim 3 inwhich the metal fluoride is generated in situ from a metal and hydrogenfluoride.
 7. A process as claimed in claim 1 which is carried out at atemperature in the range from about -b 30° C. to about 200° C.
 8. Aprocess as claimed in claim 1 wherein hydrogen fluoride is present andthe ratio of hydrogen fluoride to said compound is in the range fromabout 1:2 to about 50:1.